A conventional revolute or a simple swivel joint is shown in FIG. 1. The conventional joint has one degree-of-freedom, since members 101 and 103 are only allowed to rotate about a single axis 102. An example of a simple swivel joint is the human elbow.
FIG. 2 depicts a conventional double revolute robotic joint, which utilizes two actuators. The bottom actuator orients the bending plane 95 by rotating the whole assembly about axis 91a, and the top actuator bends the top member 92 within that plane by rotating member 92 about axis 92a. This joint has two independent degrees-of-freedom: orienting and bending. Since each degree-of-freedom is actuated by a different motor, the degrees-of-freedom are said to be decoupled. As it would be apparent to a person skilled in the art, the orientation of the top member 92 denoted by vector 94 changes as member 91 is rotated about axis 91a. The type of joint shown in FIG. 2 is not adequate for use in a snake robot because of the relative rotation between members 91 and 92. Moreover, these joints present the disadvantage of being bulky since the top actuator is installed along axis 92a thus enlarging the joint size.
Shown in FIGS. 3a and 3b is a prior art angular swivel joint. This joint is similar to the revolute joint except that the axis 113 of the joint is not perpendicular to either one of axis 111a and axis 114a of members 111 and 114, respectively. Because of this, the axis 114a of the free member 114 defines a cone of revolution 117 as it rotates about axis 113, as shown in FIG. 3b. 
FIGS. 4a and 4b depict a conventional double angular swivel joint. This joint has two motors: the bottom motor rotates the whole assembly about axis 111a and the top motor rotates the top part 114 about axis 113. Thus, the top member sweeps a cone 117 as shown in FIG. 4a, and the bottom motor rotates that cone as shown in FIG. 4b. Since there are two actuators, this joint has no more than two degrees-of-freedom, and like the simple double revolute joint described above, the two independent degrees-of-freedom are bending and orienting. Orienting is achieved by actuating the bottom motor. But, in order to achieve bending, the two motors must be actuated simultaneously in such a way that the top member stays in one plane 115. Hence, the bending degree-of-freedom is coupled between two motors. Like in the simple double revolute joint described above, the orientation 118 of the top member changes to 118′″ which in not parallel to 118. This means that relative rotation has occurs between the two members, and thus the double angular swivel joint cannot be used as a snake robot joint.
Two degrees-of-freedom joints suitable for snake robots have been the subject of much research. The first generation of designs includes simple double revolute joints connected to one another to form a snake robot. Such designs afford the robot limited capabilities and size. A more advanced design is the actuated universal joint. However, this design has the disadvantage of being bulky and has the problem of twisting. Yet another design uses an angular double swivel joint as a subassembly. This design has irregularity in the universal joint and has a relatively small strength-to-size ratio.
The NEC Corporation snake robot is one of the first designs that use an angular double swivel joint as a subassembly (U.S. Pat. No. 4,683,4061) and is shown in FIGS. 19a and 19b. This design utilizes a relatively large universal joint to prevent any relative twist between the two bays. Nonetheless, this joint is heavy and bulky. Moreover, since the universal joint does not transfer rotational speed between the two members linearly, the joint is difficult to control and the bending speed is not constant.
The prior art also includes the JPL serpentine robot built at NASA's Jet Propulsion Laboratory and the California Institute of Technology shown in FIGS. 20a and 20b. This design is based on the NEC Corporation robotic joint mentioned in the immediately preceding paragraph The JPL serpentine robot is 1.5 inches in diameter, approximately 3 foot long, weights 6 lbs., and has 10 degrees-of-freedom. All joints are direct-drive motor controlled, and all motors are mounted internally. This design also uses a universal joint, which, in contrast with other designs, is mounted inside the robot. Because the joint in mounted inside the robot, the size of the joint is limited and must be relatively small. Moreover in the JPL robot, the universal joint is necessarily hollow in order to run electrical connections through the snake robot. This small and hollow universal joint has the disadvantage of being weak, breaking easily, and exhibiting important backlash and slack.
The Pacific Northwest National Laboratory (PNNL) designed a 14 degrees-of-freedom aim, which differs from the robots descried above in that it does not use the double angular swivel joint as a subassembly. It is shown in FIGS. 21a and 21b. This design uses a simple actuated universal joint similar to the double revolute joint described in FIG. 2. However, this design uses threaded screws to actuate the universal joint making the joint strong but at the cost of being very slow. Additionally, this joint lacks robustness.
With the exception of the joints used in the NEC corporation, JPL and PNNL snake robots described above, two degrees-of-freedom joints of the prior art cannot be used in a snake robot. Because there is relative twisting between the members, electrical connections running along the body of the robot may be severely damaged or destroyed. This relative twisting may be substantially reduced by introducing a third actuator for maintaining constant orientation. However, controlling a third actuator can be very complex and substantially adds to the cost and size of the joint.
What is needed is a swivel joint that utilizes only two actuators to extract two degrees-of-freedom, wherein the joint's orientation is maintained, thus making the joint adequate for use in a snake robot.
Features and Advantages of the Joint Design in Accordance with the Present Invention
Compactness
The joint in accordance with the present invention has a highly compact design compared to prior art snake robot joint designs, and uses conventional parts (mostly off the shelf) and simple machining. The nominal diameter of the bays is preferably around 1.6 inches and 1.85 inches along the joint. The link or stage length is about 6.5 inches. These dimensions are restricted by the fact that off-the-shelf components are used in a preferred embodiment. However, it will readily appear to a person skilled in the art that a smaller or larger joint may be fabricated according to the particular applications the joint is designed for. In fact, the joint of the present invention could be as small as practicable or as big as necessary. The dimensions disclosed above are for the sole purpose of illustrating a preferred embodiment and are in no way meant to be limiting. Designs which do not use off-the-shelf components will naturally come with a greater cost due to the necessity to machine custom made components.
Strength
An important feature which set the present invention apart from the prior art is the use of angular bevel gears. These gears mate on the periphery of the joint diameter, and thus are capable of transmitting high forces and withstanding high torques. Moreover, bearings are preferably chosen to withstand very high loads, preferably up to 15 Newton-Meter torque or more depending on the application, and are preferably positioned such that they accommodate most high forces which the joint is subjected to and diminish these forces before transmitting them to the gears. In fact, the bearings take all the forces and torques that the joint faces, and the gears are only responsible for preventing relative twisting between the bays, which is a relatively minimal load. Additionally, the present joint has a high overall mechanical advantage, which allows the use of small motors and low torques. This prevents the need for expensive custom made motors. This strength of the joint is critical in all kinds of self-locomotion, climbing, shoring and other applications. In a preferred embodiment, the joint is capable of lifting up about half of the entire robot off the ground. For example, in a snake robot formed of 11 bays, the joint is capable of lifting up 6 bays. Naturally, a snake robot in accordance with the present invention may comprise as many bays as necessary.
Rolling Capability
The joint of the present invention has an additional degree-of-freedom compared to prior art snake robot joints. When the joint is in the straight position, the upper and lower cups form a circular profile. With the gear train, the upper and lower cups can be rotated as one rigid body, and the gear train can thus be used as a wheel which may be utilized to create a third degree-of-freedom. The only constraint for this added feature is that the joint be in the straight position. Therefore, this added feature is preferably only used on relatively smooth straight surfaces. In accordance with this invention, by only using two actuators, “two plus one” degrees-of-freedom can be extracted. It is to be noted, that the present joint does not have three degrees-of-freedom in the strict definition of the term, since the third degree-of-freedom is only available in the special case where the joint is in the straight position.
Reachability
A unique feature the joint of the present invention its reachability. The present invention joint has a reachability of 180 degrees. Such reachability has not been achieved in the prior art. The present joint can bend by 90 degrees in each direction, so the range of first degree-of-freedom goes from −90 to +90 degrees. The range of the second degree-of-freedom goes from 0 to 360 degrees. Thus, fixing one bay, the second bay can reach any point on a complete hemisphere as shown in FIG. 22. Moreover, since the joint has a hollow assembly and further there is no relative twisting between the bays, the snake can move from one configuration to another, smoothly, quickly and efficiently, without the need to reset the joint to the straight position as is the case with many prior designs.
Flexibility
Another unique aspect of the joint of the present invention is its flexibility. Unlike prior art two is degrees-of freedom joints, the present joint has infinite flexibility as illustrated in FIG. 22. This means that starting from any point on the hemisphere the free end can start moving in any arbitrary direction. In other words the tangent space of the free arm is a plane which is tangent to the hemisphere. This feature is particularly important in snake robot design, since it allows the snake to move “directly” from any configuration to another (using the shortest path) in minimal time and with minimal power consumption. This feature is also particularly important for applications where the robot is restricted to least interference with the environment.
Hollow Shaft Assembly
In a snake robot built with a plurality of joints in accordance with a preferred embodiment of the present invention, a hole with a diameter of about preferably 0.1 to 0.5 inch, most preferably 0.3 inch, goes through the entire length of the body of the snake. Smaller and larger diameters may also be appropriate as well as hole having, any appropriate shape know in the art. This is critical to snake robot design, since it allows to run electrical connections inside the snake, and these electrical connections are protected from the external environment. For example, one of many applications of a snake robot is search and rescue operation in collapsed buildings (e.g., after an earthquake) where a harsh, rough and dirty environment awaits the robot. Moreover, the hollow shaft can receive and protect other needed connections, such as, for example, optical fibers.
Orientation Preserving
In the present design, the bays bend in the desired configuration without any relative twist between the bays. This is critical for running wires through the inside of the snake without risking to damage the wires. In this manner, wires only bend, but would not twist, which is mechanically safe for electrical wires. Thus, a snake robot in accordance with the present invention may go from a given configuration to another very quickly and smoothly without any concern for mechanical failure of the wires. In other words the motors can be continuously actuated for any desired mode of motion, such as bending or orienting or any other mode. In particular we can actuate our joint to be in orientating mode when it is in the straight position. In this case (straight position) a third degree-of-freedom is available: rolling. Thus, the present novel mechanism is able to produce three modes of motion with a simple controller, such as, but not limited to, a simple on/off switch. The controller's functions only need to be as simple as actuating the motors with the same speed but with equal or different orientations. The joint can be actuated by simply turning the motors on or off, and no complicated controller to continuously vary the speed of the motors is needed.
Linear Control
In a preferred embodiment of the present invention, there is a linear relation between the motor speed and the azimuth and elevation angles. Thus, a simple linear controller may be used. The motors are preferably run in a continuous linear mode in order to produce continuous smooth bending and orientation. This is due to an angular bevel mechanism, which provides continuous motion between the bays. In contrast, the universal joint mechanisms of the prior art have a non-constant motion transfer, which leads to complex control and non-smooth behavior or the joint.